Method of designing a non-progressive multifocal ophthalmic lens

ABSTRACT

The present invention relates to a multifocal ophthalmic lens which comprises a first substantially spherical area for distance viewing, a second substantially spherical area for near viewing having a desired width and height and being surrounded on a plurality of sides by the first area, and relatively narrow areas intermediate the first and second areas for blending the second area into the first area. The lens design of the present invention may also include an optional third substantially spherical area adjacent the second area for increasing the near viewing range in a substantially contiguous manner.

CROSS REFERENCE OF RELATED APPLICATION

This is a divisional application of U.S. patent application Ser. No.190,149, filed May 4, 1988 now U.S. Pat. No. 4,869,588 which is acontinuation-in-part application of U.S. patent application Ser. No.095,891, filed Sept. 14, 1987, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to ophthalmic lenses for use in eye wear.

Multifocal lenses have been used in situations requiring spectaclecorrection for presbyopia. Often patients using such correction devicesencounter disturbing problems inherent in many designs. These problemsinclude image jump, limited range of clear vision, annoying reflectionsfrom the edge of lens segments and cosmetic objections. For example,conventional flat top, curve top and ribbon shaped segments have ledgeswhich either protrude from the plastic lenses or are fused within theglass lenses. These ledges often reflect light into the wearer's eye andare cosmetically unappealing because of their obviousness to theobserver.

Attempts have been made to correct these problems through the use ofround segments. Such segments, however, still provide a clearlynoticeable, cosmetically unattractive boundary between the near anddistance areas. Many round and flat top segments provide clear visionfor distance and near viewing but fail to overcome the problem of imagejump when passing from the near viewing zone to the distance vision zoneand vice versa. Several early attempts to deal with these problems areshown in U.S. Pat. Nos. 1,448,052 and 1,518,405.

In recent years, progressive lenses have been used to correctpresbyopia. Progressive lenses besides their obvious cosmetic appealprovide a continuous range of focal powers. This benefit is partiallyoffset by the peripheral astigmatism and distortion abberations that areunavoidably present in almost all progressive lenses. The design ofmodern progressive lenses often centers on reducing these unwanteddistortions. U.S. Pat. Nos. 3,687,528, 3,711,191, 4,461,550, 4,484,804,4,514,061 and 4,592,630 illustrate some of the progressive lenses knownin the art.

One of the deficiencies of currently available progressive lenses isthat they do not provide sphericity for near viewing. Instead they uselarge numbers of radii in an attempt to give a gradual progression invertically increasing power. They make the assumption that it isnecessary to provide clear vision for all near working distances. Theproblem which arises from this is that each progressively shorter radiusleaves an area of uncorrectable distortion as it departs from theconnecting point which is the vertical center line of the segment.Vision with standard progressive lenses is conceptually clear at thecenter line of the lens. In practice, however, the human eye typicallyrequires an area at least 18 to 35 mm wide for horizontal field comfort.

Some progressives also include blended areas which extend all the way tothe edges of the lens. These blended areas are intended to minimizedistortion and achieve cosmetic appeal. The erroneous assumption made bythe designers of such lenses is that patients can tolerate distancedistortion. Still another deficiency of many progressive lenses is thatthey have narrow corridors of power increase which are difficult foropticians to correctly position, especially for reading. It is alsodifficult for the user to aim the lens toward the reading area.

Accordingly, it is an object of the present invention to provide anophthalmic lens which provides clear viewing substantially withoutdistortion in both near and distance viewing areas.

It is a further object of the present invention to provide a lens asabove having an extended near viewing range when required.

It is a still further object of the present invention to provide a lensas above in which there is substantially no image jump.

It is yet another object of the present invention to provide a lens asabove which is cosmetically appealing and has no visible transitionlines.

It is yet another object of the present invention to provide a lens asabove which can be easily fitted to the user.

These and other objects and advantages will become clearer from thefollowing description and drawings.

SUMMARY OF THE INVENTION

The foregoing objects are achieved through the design and use of amultifocal ophthalmic lens which comprises a first substantiallyspherical area for distance viewing, a second substantially sphericalarea for near viewing having a desired width and height and beingsurrounded on a plurality of sides by the first area, and relativelynarrow areas intermediate the first and second areas for blending thesecond area into the first area. The lens design of the presentinvention may also include a third substantially spherical area adjacentthe second area for increasing the near viewing range in a substantiallycontiguous manner.

To accommodate the natural inward turning of the eyes when viewingobjects at a near distance, the near distance viewing area(s) arepreferably inclined nasalward. The many advantages attendant to thisdesign include minimal areas of distortion resulting from the blendingof various lens segments, the relative absence of distortion in the longdistance viewing area, an extended near vision viewing range, and easeof fit for a dispensing optician.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a front view of a lens in accordance with the presentinvention.

FIG. 2 illustrates a side view of a lens designed in accordance with thepresent invention.

FIG. 3 illustrates a front view of a bifocal lens formed in accordancewith the present invention.

FIG. 4 is a sectional view taken along line A--A in FIG. 3 and shows theblending of the lens areas.

FIG. 5 is a sectional view taken along line B--B in FIG. 3 and shows theblending of the lens areas.

DETAILED DESCRIPTION

As previously discussed, the present invention relates to a multifocalophthalmic lens for providing correction for presbyopia. Lenses inaccordance with the present invention provide the desired correctionwithout encountering problems such as image jump, annoying reflectionsfrom segment edges and large amounts of distortion. In addition, lensesin accordance with the present invention are cosmetically appealing.

Referring now to the drawings, FIG. 1 illustrates a multifocal lens 10in accordance with the present invention. The lens 10 is characterizedby a first substantially spherical area 12 for distance viewing, asecond substantially spherical area 14 for near viewing, an optionalthird substantially spherical area 16 for extending the viewing rangeprovided by the second area in a substantially contiguous manner, andareas 18 for blending the second and third areas into the first area.

The sphericity of the areas 12, 14 and/or 16 is important for a numberof reasons. First, spherical or substantially spherical viewing areasare particularly desirable from the standpoint of providing clear,undistorted areas of vision in both near and far viewing ranges. Second,the spherical construction of the viewing areas combined with the uniquemethod of connection described herein permit the lens to be fabricatedwithout the cosmetically unappealing lines or ledges which appear inmany conventional bifocal and trifocal designs.

As can be seen from FIG. 1, the relatively narrow blend areas 18 arelocated on the sides of the areas 14 and 16, at portions of the top ofarea 16 and at the bottom of the near distance viewing area 14. In mostcases, these blend areas will have a width of less than about 10 mm,preferably less than about 6 mm. This is also quite significant in termsof providing a lens with relatively little distortion. It should also benoted that the lens of the present invention differs from manyprogressive lenses in that the blend areas do not extend to the lensedges. As will be described in more detail hereinafter, the blend areas18 may be produced using any suitable technique known in the art.Preferably, the curvature of these areas is determined through the useof sinusoidal curves.

One of the real advantages attendant to the lenses of the presentinvention is that the depth, width and shape of the areas 14 and 16 canbe arbitrarily chosen for convenience and patient comfort. For example,the width of each near distance viewing area may be in the range of fromabout 18 mm. to about 45 mm. and the depth of each area may be in therange of from about 11 mm. to about 28 mm. It is preferred, however,that each of the areas 14 and 16 have a substantially flat top portion.As can be seen from FIG. 1, the long distance viewing area 12 surroundseach of the areas 14 and 16 on a plurality of sides, preferably at leastthree sides.

The intermediate viewing or third area 16 is truly optional in that itmay not be needed at all for a patient. Bifocal ophthalmic lenses suchas that shown in FIG. 3 may be formed using the same designconsiderations, e.g. sphericity, sinusoidal blending, etc. as thosedescribed in connection with lenses having three viewing areas. Such abifocal lens has a first substantially spherical area 12' for distanceviewing, a second substantially spherical area 14' for near viewinghaving a desired width and height and surrounded on one or more sides bythe first area and relatively narrow areas 18' intermediate the firstand second areas for blending the second area into the first area. Thenear viewing area 14' of the bifocal lenses formed in accordance withthe present invention may have any desired add power. It is prefered inthe bifocal lens however that the near viewing area 14' have asubstantially flat top portion.

The technique employed to sinusoidally blend the distance viewing area12' and the near viewing area 14' is illustrated in FIGS. 4 and 5. Thearea 12' is defined by a first arc having a radius R₁ while the area 14'is defined by a second arc having a radius R₂ different from the radiusR₁. For reasons which relate to the principles behind the method ofconnection of the arcs and which are described in more detailhereinafter, the two arcs substantially coincide for some distance. Thiscoincidence negates the need to blend the very top portion of the area14' into the area 12'. Other portions where the two arcs do not coincideare blended by areas 18'. These areas adjacent the top portion of thearea 14' are blended by drawing two arcs 30 and 32, each having the sameradius. The radius is that of the maximum curve that contacts a segmentof a viewing area and the other arc. The arc 30 is drawn on one side ofthe lens while the arc 32 is drawn on the opposite side with one arctangent to a portion of the arc defining viewing area 12', the other arctangent to a portion of the arc defining area 14', and the two arcs 30and 32 tangent to each other. Since the two arcs 30 and 32 have the sameradius, they will define a sinusoidal blend area between the areas 12'and 14'. As can be seen from FIG. 5, the areas 12' and 14' are blendedalong the sides in a similar fashion using arcs 34 and 36. The radii ofthe arcs 30, 32, 34 and 36 used to blend the areas 12' and 14' differfrom point to point around the area 14'. They are chosen howeverconsistent with the above considerations and the principles and desiredgoals outlined herein. The result is a bifocal lens having a nearviewing area integrated into a distance viewing area.

When it is determined that a lens must have an intermediate viewing area16, the power required for the intermediate area can be determined bybeginning with the prescription that is normally prescribed for abifocal addition. From this the near and far range of vision for theintermediate viewing or third area 16 can be determined from thefollowing equations: ##EQU1## where Add=The add prescribed forcomfortable reading between 14" to 16";

U.A.=The total amount of usable accommodation which is +2.50 minus thereading add; and

D.A.=Depth of focus which is a constant +0.25. Therefore, if a patientreceives a prescription for a +2.00D. add, the far range is 20" and thenear range is 14.6". In using equation (2), it should be noted that U.A.can never be less than zero. Therefore, for prescribed adds, greaterthan 2.5, the U.A. is always zero because there is no visualaccommodation left.

As previously discussed, the primary purpose of the intermediate viewingarea 16 is to extend the viewing range provided by the near viewing area14 in a substantially contiguous manner. This may be done using thefollowing table.

                  TABLE 1                                                         ______________________________________                                                         % of Add Used                                                                 for Second  Inter-  Total                                    Add              (Upper) Inter-                                                                            mediate Extended                                 Bifocal                                                                              Range,    mediate/Area                                                                              Range,  Range,                                   Power  In.       Actual Power                                                                              In.     In.                                      ______________________________________                                        +1.00  14.6-40   --/--       --      14.6-40                                  +1.25  14.6-40   --/--       --      14.6-40                                  +1.50  14.6-26   66%/+1.00   17.7-40 14.6-40                                  +1.75  14.6-22.8 57%/+1.00   20-40   14.6-53                                  +2.00  14.6-20   62.5%/+1.25 20-32   14.6-32                                  +2.25  14.6-17.7 77.7%/+1.75 17.7-22.8                                                                             14.6-22.8                                +2.50  14.6-16   80%/+2.00   17.7-20 14.6-20                                  +2.75  13.3-14.6 81.8%/+2.25 16-17.7 13.3-17.7                                +3.00  12.3-13.3 83.3%/+2.50 14.6-16 12.3-16                                  ______________________________________                                    

By locating the prescribed bifocal add power, one can determine thepower needed for the intermediate area 16.

The following example illustrates how a lens similar to that shown inFIG. 1 is designed. With reference to FIG. 2, assume the lens is to havea +6.00D. front base curve with a +2.00D. bifocal add. The radius R₁ forthe 6.00D. curve is determined by dividing the constant 530, thestandard index of refraction of optical glass, by 6. Thus R₁ equals83.33 mm. An arc 20 representing the distance viewing area 12 is drawn.The arc of course has a central axis A--A. The intermediate area 14begins at a desired distance, preferably about 3 mm, below the centralaxis A--A. At this point, a tangent T₁ is constructed. A line B--Bperpendicular to the tangent T₁ is also constructed. The center C₂ forthe radius R₂ of the intermediate section lies along the line B--B.Since the bifocal add is +2.00D., the power of the intermediate area 16can be determined using Table I. In this case, the intermediate power is+1.25D. To obtain the radius R₂ of the intermediate area, the standardindex of refraction 530 is divided by the sum of +1.25D. and +6.00D.Thus, R₂ equals 73.1 mm. At point 73.1 mm from the intersection oftangent T₁ and line B--B along line B--B forms the center C₂ of theintermediate area. A second arc representing the intermediate area isthen drawn from the tangent T₁ to a point approximately 7 mm below. Atthis point, a second tangent T₂ is drawn and another perpendicular lineC--C is constructed. The point defined by the intersection of thetangent T₂ and line C--C represents the upper limit of the near fieldviewing range 14. The radius R₃ of curvature for the area 14 isdetermined by dividing 530 by the sum of the +2.00D. add, the prescribedbifocal add, and +6.00D. The radius R₃ is thus 66.25 mm. The center C₃of the area 14 is determined by moving a distance 66.25 mm along theline C--C from the tangent T₂. The area 14 comprises an arc beginning atthe tangent T₂ and traveling downward a desired distance, approximately13 mm. The point T₃ which is 13 mm below the tangent T₂ determines thebottom of the near field viewing area. As shown in FIGS. 1 and 2, thisis then blended into the rest of the lens determined by the arc 20defined using radius R₁.

It has been found that by using this design technique the adjacent arcsdefining the viewing areas 12, 14 and 16 nearly coincide for somedistance. It is the coincidence of these lines that eliminates thetransition lines normally seen in bifocals and trifocals. This nearcoincidence also negates the need to blend the upper portion of thesegments 14 and 16 to the next adjacent portion. Thus, blending islimited to the sides of the areas 14 and 16, portions of the top of area16, and the bottom of area 14. At these points, blending curves are usedto connect the short radii to the longer radius of the distance curve.While it is preferred to use sinusoidal blending curves as show in FIG.3, contangent and/or any other set of blending curves known in the artcould be used with the same results. The object is simply to connect thenear and intermediate viewing segments 14 and 16 to the distance visionarea 12 in as narrow an areas as possible with the aesthetic advantagesof invisibility having an extremely high priority so that there will beno visible transition lines. It should be noted from FIG. 2 that thecenters C₂ and C₃ of the areas 16 and 14, respectively, do not lie alongthe central axis A--A.

In accordance with the foregoing, it can be seen that lenses designed inaccordance with the present invention provide:

(a) a relatively wide area of increased power for near vision withsubstantially no image jump;

(b) a completely spherical and clear distance vision area outside theblended near area boundary lines;

(c) a lens with clear distance vision and one or two near viewing areasbut having no visible transition lines;

(d) a lens with an increased powered area for near viewing and cleardistance vision for peripheral vision and spacial orientationinferiorly;

(e) a lens with an increased powered near area which is easy tocorrectly position for the dispensing optician; and

(f) a lens with substantially spherical reading and extended near closework areas.

In addition, the lens of the present invention may have a standardthickness. This further distinguishes the lens 10 from progressivelenses which tend to be thicker at the center.

The lens of the present invention could be made from either glass orplastic using any suitable fabrication technique. For example, the lenscould be fabricated using a ceramic platform or a metal mold producedusing computer directed numerically controlled pantograph machining. Themetal molds may be used to cast plastic lenses while the ceramicplatforms may be used for glass molds which cast lenses. If necessary,the lenses can be finished using standard grinding and polishingtechniques. Further, if desired, the finished lens can be bonded to asingle vision distance prescription lens.

Preferably, the segment 22 formed by areas 14 and/or 16 is inclinednasalward as one looks downward to view at near working distances. Forexample, the center C₃ of the near area 14 may be 2 mm nasalward fromthe center C₁ of the distance area 12 to accommodate for the naturalinward turning of eyes when viewing objects at a near distance.

It is apparent that there has been provided in accordance with thisinvention an ophthalmic lens which fully satisfies the objects, means,and advantages set forth hereinbefore. While the invention has beendescribed in combination with specific embodiments thereof, it isevident that many alternatives, modifications, and variations will beapparent to those skilled in the art in light of the foregoingdescription. Accordingly, it is intended to embrace all suchalternatives, modifications, and variations as fall within the spiritand broad scope of the appended claims.

What is claimed is:
 1. A method of designing a non-progressiveophthalmic lens having a distance viewing area and at least one otherviewing area, said method comprising:forming a first arc segmentdefining a portion of a substantially spherical far distance viewingarea having a prescribed power; forming a second arc segment defining aportion of a second viewing area having a desired power and a topportion coinciding with said first arc segment for a first distance; andblending said first and second arc segments together in areas where saidsegments do not coincide, said lens being characterized by substantiallyno image jump, no visible line segment at said top portion and noseparation through a center region of said top portion.
 2. The method ofclaim 1 wherein said first arc segment forming stepcomprises:establishing a first center of curvature for said far distanceviewing area; determining a first radius of curvature for said fardistance viewing area based upon said prescribed power for said fardistance viewing area; and drawing said first arc segment defining saidfar distance viewing area, said first arc segment having a central axispassing through said first center of curvature.
 3. The method of claim 2wherein said second arc segment forming step comprises:selecting a firstpoint along said first arc segment at a desired distance below saidcentral axis, said first point defining the top portion of the secondarc segment; drawing a first line tangent to said first arc segment atsaid first point; drawing a first line perpendicular to said firsttangent line; determining a second radius of curvature for said secondviewing area; establishing a second center of curvature by measuring adistance equal to said second radius along said first perpendicularline; and drawing said second arc segment for a desired distance.
 4. Themethod of claim 3 further comprising:forming a third arc segmentdefining a portion of a third viewing area having a desired power, saidthird arc segment having a top portion coiciding with said second arcsegment for a second distance.
 5. The method of claim 4 wherein saidthird arc segment forming step comprises:selecting a second point alongsaid second arc segment at a desired distance below said central axis,said second point defining the top portion of the third arc segment;drawing a second line tangent to said second arc segment at said secondpoint; drawing a second line perpendicular to said second tangent line;determining a third radius of curvature for said third viewing area;establishing a third center of curvature by measuring a distance equalto said third radius of curvature along said second perpendicular line;and drawing said third arc segment for a desired distance.
 6. The methodof claim 1 wherein said blending step comprises sinusoidally blendingsaid arc segments together in areas where they do not coincide.